Optically clear adhesives (hereinafter referred to as “OCA”) are widely used in order to bond constituent components of optical laminates such as touch panels, for example, a surface protection material (cover lens) to a touch panel module. Display units provided with touch panels can be used in a variety of environmental conditions, such as high temperature and/or high humidity depending on the intended use of the display unit. In addition to maintaining high transparency under such environmental conditions, OCAs need to exhibit characteristics such as heat resistance and resistance to moist heat. These characteristics can be evaluated by investigating whether or not bubbling, peeling, clouding or white opacity or the like occurs in an optical laminate when an optical laminate that contains an OCA is subjected to an accelerated aging test under conditions of high temperature/high humidity.
Bubbling and peeling, which can occur in optical laminates, are caused by outgas produced from materials that constitute optical laminates, differences in coefficient of thermal expansion between constituent materials, and the like. For example, plastic films used as cover lenses for touch panel modules, such as poly(methyl methacrylate) (PMMA) or polycarbonates (PC), can produce outgas under high temperature conditions, thereby leading to bubbling or peeling between these films and a touch panel module. Meanwhile, under moist heat conditions, moisture vapor from the outside can penetrate from the sides of a touch panel, pass through these films or condense inside a touch panel when the touch panel is cooled, thereby causing clouding or white opacity inside the touch panel.
In terms of the structure of a touch panel, OCAs need to not cause loss of electrical conductivity in cases where an OCA can come into contact with an electrically conductive layer that forms a circuit on a touch panel. Electrically conductive layers are formed from metals or metal oxides, such as gold, silver, copper, aluminum, indium tin oxide (ITO) or antimony tin oxide (ATO), and there are concerns that the resistance of an electrically conductive layer will increase as a result of corrosion if the electrically conductive layer comes into contact with an acidic component.
In recent years, plastic panels such as poly(methyl methacrylate) (PMMA), polycarbonates (PCs) and cycloolefin polymers (COPs) have been used as touch panel modules for automotive applications in particular, due to being lightweight, flexible, inexpensive, safe, and so on. Both conventional cast molded articles and in-mold molded articles are used in touch panel modules having three-dimensional shapes. In cases where an OCA is used to bond a cover lens to a touch panel module that includes this type of plastic panel or molded article, both the cover lens and the plastic panel or molded article are involved in the generation of outgas and the transmission of moisture vapor, and it is therefore difficult to prevent bubbling, peeling, clouding or white opacity in a touch panel. In addition, touch panels designed to be outdoors for long periods of time, such as automotive touch panels, require superior heat resistance and resistance to moist heat compared to conventional touch panels.
Patent Document 1 (Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2012-504512) discloses “an optically clear laminate comprising: a first base material having at least one major surface; a second base material having at least one major surface; and a cloud point-resistant, optically clear adhesive composition situated between and in contact with at least one major surface of the aforementioned first base material and at least one major surface of the aforementioned second base material, wherein the aforementioned adhesive composition is derived from precursors that comprise: from about 60 to about 95 parts by weight of an alkyl acrylate having 1 to 14 carbon atoms in the alkyl group; from 0 to about 5 parts of a copolymerizable polar monomer; and from about 5 to about 50 parts of a hydroxyl-containing monomer that has an OH equivalent weight of less than 400”.
Patent Document 2 (WO/2010/040014 (Japanese Unexamined Patent Application Publication No. 2013-018227)) discloses “a laminate has a base material, an tacky adhesive layer; and a conductive layer, laminated in this order, wherein the aforementioned tacky adhesive layer contains (A) a (meth)acrylic polymer, which has a weight average molecular weight of 50 thousand or higher and less than 400,000, contains substantially no acid groups and is obtained by copolymerizing 0.1 to 10 wt. % of a (meth)acrylic monomer having a crosslinkable functional group and 0.1 to 30 wt. % of a macromer (relative to 100 wt. % of the total monomer quantity), and (B) a crosslinking agent, wherein the aforementioned macromer has a polymerizable unsaturated group, has a glass transition temperature of 50 to 180° C. and a weight average molecular weight of 500 to 100,000”.